1. Field of the Invention
The present invention relates to a process and apparatus for signal analysis, and more specifically, for measuring modulation and non-linearities, for both continuous wave (C.W.) and ramped signals from a unit under test, such as a radar system.
2. Description of the Related Art
Prior art methods and devices for measuring modulation and non-linearties for linearly swept signals employ a delay line mixer scheme which provides a signal whose output is sensitive to the swept signals' slope and linearity. The frequency output signal of the mixer is proportional to the signal phase difference between the delayed and undelayed signals. The prior art method is generally embodied with some specific details as follows.
For radio frequency (RF) microwave signals containing frequency modulation (FM), the signal is frequency down-converted to a predetermined IF frequency. The IF signal has the FM characteristics originally present in the microwave output signal of a unit under test. The IF signal is then divided and part of the signal is input into an expensive precision delay line. The amount of delay is selected such that when the delayed signal is mixed with the original undelayed IF signal, a precise 2 KHz difference frequency will be output from the mixer (assuming the slope of the FM signal, delay line, and signal linearity are ideal). For different signal sweep rates, a different delay line is used. It should also be noted that for signals that are not at RF or microwave frequencies, no down-conversion is required. The signal is divided and fed into the delay line and the mixer combination directly.
The 2 KHz difference frequency signal is then converted into a form suitable for analysis by digital techniques employing amplification and limiting. Half of each 2 KHz signal pulse period digital signal is measured by using a 256-bit elapsed counter circuit. The value measured by the elapsed counter circuit is then stored. The stored result of the counter is then converted from a digital signal to an analog signal by an 8-bit (2.sup.8 =256) digital-to-analog converter for display on an oscilloscope. For example, if the swept signal ramp is 8 milliseconds in duration, there will be sixteen 2 KHz pulses which will be measured and displayed by the digital-to-analog converter. The digital-to-analog converter is updated for every 2 KHz signal pulse period.
If the signal ramp is linear and the reference signal frequency is less than the ideal 2 KHz, then at each period the phase difference increases by the same amount. The display is then an increasing staircase signal with equal-sized steps. As the reference frequency is increased, the steps become smaller in size until the display is a flat line.
The curvature of the signal representation on the display (with respect to a horizontal presentation) determines the non-linearity of the sweep. As the reference signal frequency is increased still more, the display becomes a downward staircase signal. A human operator adjusts the reference frequency signal until the last displayed data point intersects zero frequency on the display, and then reads the reference frequency on a frequency counter. The frequency measured is proportional to the average slope of the ramp of the signal. If the measured frequency is outside of the allowable frequency range, then the signal slope is not within prescribed specification.
Therefore, prior art inventions for swept signal linearity testing are generally performed with specially designed test panels. The test panels use a sophisticated technique of swept signal detection and analysis. This detection technique requires the use of an expensive precision delay line with expensive circuitry with substantial human operator interaction.
There are several disadvantages to the prior art technique. First, the precision delay line requires about forty five minutes to warm up for operation before measurements can be made. Second, the determination for pass or fail of the swept signal ramp is done manually with human operator intervention. Such intervention causes the measurements to be subjective and the accuracy of the measurements to rely upon skill of the operator. Third, even with skilled operators, the above described procedure is cumbersome and lengthy. Furthermore, the hardware used for prior art devices is special purpose, relatively inaccurate, and has limited adaptability for other uses.
Recent market requirements for desired signal test equipment, as in the radar test equipment art, dictate the need for automatic test equipment. Salient features of such desired equipment are: smaller size and lighter weight for depot, intermediate and organization testing; mobile and transportable; a high degree of automation (with minimal human operator intervention); and low cost.
Consequently, it would greatly benefit the art by employing an automated digital signal processing method and apparatus which would operate rapidly and would be inherently more repeatable, more accurate, and lower in cost.